1. Field of the Invention
The present invention relates to a method of inspecting any crack in a ceramic substrate, and in particular, a ceramic substrate used in manufacturing a sensor, a resistor element, an actuator for an ink-jet print head, or the like.
2. Description of Related Art
Conventionally, a ceramic substrate coated with a conductive film has been used as an electrode, a resistor, a lead, an output terminal, or the like in a sensor, a resistor element, an actuator for an ink-jet print head, or the like.
As a method of manufacturing a ceramic substrate coated with a conductive film, a method of applying by printing conductive paste on a ceramic substrate is appropriate. This is because a uniform film having a complicated pattern can be efficiently formed and the conductive film can be adjusted to have a wide range of thickness by adjusting the viscosity of the conductive paste.
In order to meet the recent demands for a miniaturized and lightened actuator for an ink-jet print head, or the like., thinning of a ceramic substrate has been carried forward, and recently, a ceramic substrate which is 10.mu.m or less in thickness is now available. This is accompanied by a fear that handling of a ceramic substrate in manufacturing may cause a crack in the ceramic substrate. If a ceramic substrate is cracked, not only the mechanical strength of the ceramic substrate is so lowered that it can not be put to practical use, but also leakage of ink may be caused.
As a method of inspecting any crack in a ceramic substrate, conventionally, there are known a red check test, a method of inspection by visual observation or with microscope using liquid penetrant (liquid penetrant test) such as a fluorescent penetrant test, a method of inspection by detecting leakage of gas such as gaseous helium and air, a method using an image processor, etc.
However, the liquid penetrant test suffers from a problem that it depends to a great extent on the skillfulness of the inspector, cracks are likely to be left undetected, and the result of the inspection is difficult to express as numerical data. Further, in case a ceramic substrate applied to, for example, an actuator for an ink-jet print head has a multilayer functional film formed thereon, there is a problem that, according to the method using liquid penetrant, it takes the liquid penetrant much time to ooze.
The method of inspection by detecting leakage of gas has a problem that, since a closed space is necessary, there is technical difficulty and the scale of the apparatus has to be large.
Further, the method using an image processor has a problem that, depending on the resolution of the apparatus, a microscopic crack may be difficult to detect.
Still further, as a defect common to the conventional methods on the whole, there is a problem that a crack which is, for example, shut by a functional film such as a conductive film and thus, which is not exposed on the surface of a ceramic substrate can not be detected.